Substrates made of or containing aromatic polymers are often used in the construction of certain electronic assemblies, such as micro-electronic packaging. A large number of polymers have been found to be satisfactory for use as such substrates. Polyimides have been found to be particularly suitable in this regard, partly because of their excellent thermal stability and solvent resistance. One widely used and commercially available polyimide is sold under the trade name Kapton®, available from DuPont, Wilmington, Del. Other polyimides are sold under the tradenames Apical®, available from Kaneka High-Tech Materials and Upilex® and Upimol®, available from Uniglobe Kisco, Inc. and are also widely used.
Polyimides are extensively used in micro-electronic packaging applications such as flexible (Flex) circuits, rigid-flex circuits, printed circuit boards (PCB's), and multi-layer flexible circuits and as passivation layers on silicon chips. However, aromatic polymers, such as polyimides, by themselves tend to have poor adhesion with metals (such as copper, nickel and gold), which are subsequently plated thereon. In addition, when an electrolessly deposited metal is used as a conductive layer for additional deposition of a metal such as by electrolytic deposition, the adhesion between the polymer (polyimide) substrate, the electrolessly deposited metal and the electrolytically deposited metal must be sufficiently strong. Thus, it has been necessary to develop certain techniques for improving the adhesion between such metals and these substrates, and several methods have been adopted in order to attempt to overcome this problem with poor adhesion.
For example, an adhesive may be used to bond the metal layers to the polyimide film and, thereby, make metal-clad polymer films. Lithography is then used to pattern the metal layer. However, with these metal-clad films it is difficult to achieve fine line circuitry because etching of the metal layer leads to undercuts (due to etching underneath the mask) in the circuit lines and the metal layer needs to be relatively thick (at least 15 microns) to have sufficient mechanical strength for separate handling. Furthermore the adhesives used may cause difficulties in laser drilling of micro-vias.
Another method used to improve adhesion between the layers coats a liquid polyimide (or its precursor polyamic acid) onto a roughened metal foil (e.g. copper foil), followed by curing. However, fine line circuitry is again difficult to achieve due to the thickness of the metal foil.
Still another method used to improve adhesion is the sputtering of a thin layer of nickel and chromium onto the polyimide surface, followed by the sputtering of a thin layer of copper onto the chromium layer, which is thickened using electroplating. Although this method is able to produce fine line circuitry (by the use of a photoresist before the electroplating step), the sputtering steps are expensive and time consuming. The sputtering method also has the problems of the large size of its apparatus, high costs, and lower productivity as compared to other commonly used methods.
Therefore, additional efforts are needed to provide an improved method of preparing a non-conductive substrate for subsequent plating thereon that overcomes the drawbacks of the prior art.